1. Field of the Invention
The present invention relates to an X-ray generating apparatus for use with an X-ray CT scanner.
An example of a conventional X-ray generating apparatus is shown in FIG. 1. An X-ray tube 81 has an anode 82 and a cathode 83. A power-supply device 84 comprises: a high-tension transformer 85 having a primary winding 86 and secondary windings 87 and 88; rectifiers 89 and 91 for rectifying outputs of secondary windings 87 and 88; capacitors 90 and 92 for smoothing outputs of rectifiers 89 and 91; and a filament transformer 93 for supplying power to cathode 83 for heating thereof. Connected between anode 82 and capacitor 90 is a protective resistor (or an impulse resistor) 94 to limit short-circuit current which may flow when abnormal discharge take places in X-ray tube 81. A protective resistor 95 is also connected between cathode 83 and capacitor 92.
With such an arrangement, cathode 83 of X-ray tube 81 is heated by filament transformer 93, and high voltages (e.g., .+-.75 KV with respect to ground potential) are respectively applied to anode 82 and cathode 83 of X-ray tube 81 via protective resistors 94 and 95 so that thermions emitted by heated cathode 83 impinge upon anode 82 to generate X rays.
Excessive current may flow between the anode and the cathode of X-ray tube 81 because of short-circuiting thereof caused by some abnormality. The current, however, is limited by protective resistors 94 and 95. In other words, the protective resistors are useful for lengthening the life of X-ray tube 81. However, the protective resistors have a high resistance of several kiloohms and is thus large in the amount of heat resulting from current flowing therethrough. For this reason, there is the need for provision of a cooling system using oil or gas, thus disadvantageously making the power-supply device massive. Since there is the need of cooling medium, it is difficult to mold the power supply with resin. In order to reduce a weight and a size of the power-supply device, it is effective to mold the power-supply device using silicon. However, silicon molding results a poor heat-radiation.
The following problems occur when such an X-ray generating apparatus is used with an X-ray CT scanner. In the X-ray CT scanner, generally, an X-ray tube is disposed within a rotating member rotatably supported by a fixed frame, a power-supply device is placed outside the frame, and a high voltage (.+-.75 KV) is supplied from the power-supply device to the rotating member through a high-voltage slip-ring. With such a slip-ring system, there is the need of applying gas insulation or oil insulation to the slip-ring because voltage applied thereto is very high. However, it is very difficult to keep the slip-ring airtight and moreover the slip-ring itself requires a special structure. This will render the slip-ring costly.
Therefore, a low-voltage slip-ring system has been developed in which a high-voltage generator is installed in a rotating member, a low voltage (200-400 V) generated by a commercial power supply is transmitted to the rotating member through a low-voltage slip-ring, and the low voltage s converted to a high voltage (.+-.75 KV) within the rotating member. With such a system, however, since the rotating member must be equipped with the high-voltage generator comprised of a transformer, the frame itself must be made large to stand weight. As described above, the high-voltage generator cannot be made small, light and be molded. It is thus difficult to equip the frame with the high-voltage generator. Moreover, current flow must be the larger because the voltage is low. Since, however, the flow of large current (e.g., 50-200A) through the slip-ring increases the amount of heat generated by the slip-ring, it is necessary to provide a separate heat exchanger so as to prevent heat generation of the ring. In addition, where the rotating member is equipped with the high-voltage generator, the balance of the rotating member will be disturbed because the high-voltage generator is considerably heavy. For this reason, it becomes necessary to add a balance weight to the rotating member so as to keep it in balance. This will make the frame larger and larger.